1. Field of the Invention
This invention relates generally to display illumination systems, and more particularly to a light source driver and method that synchronizes light source drive pulses with the modulation of a display.
2. Description of the Background Art
It is well known that projection systems use lamps to illuminate display devices, such as a reflective or transmissive spatial light modulators (SLMs), which modulate the incident light to produce a display image. The lamps of such projection systems are often of the ultra-high pressure (UHP) type, and produce light in a series of rapid pulses.
Display devices typically include an array of pixel cells and are controlled by video signals. Each pixel cell modulates the polarity of incident light responsive to the video signals/data provided to the display device. The pixel cells of the display device are repeatedly updated in a sequential manner. For example, the pixels of the display can be updated row by row starting from the top of the display and proceeding to the bottom row of the display. After the last row of pixel cells is modulated, the process starts over. In this way, the pixel array is rapidly refreshed one frame (i.e., one full display image) of data at a time. The display refresh rates can vary according to country and manufacturer.
One problem associated with projectors of the prior art is that the light pulses produced by the UHP lamp are out of phase with the modulation of the display device. When light pulses are generated at a different rate than the rate at which the display device is refreshed, degradation of the generated image results. Furthermore, even if the generation of light pulses and refreshing of the display begin synchronized, slight variances in the light pulse generation rate of the lamp and the refresh rate of the display device cause the display refresh and the light pulses to gradually “creep” out of phase. The end result of the phase difference to the viewer is a noticeable variance in brightness of the image produced by the display device and, in some cases, unacceptable display flicker.
The problems associated with display flicker caused by lamp pulses have remained largely unresolved to date. Manufacturers have been somewhat deterred from developing a solution, because of the varying display driving standards between different countries and manufacturers. One attempted solution is described in U.S. Patent Application Publication 2004/0257326 (Chang), which discloses a method for synchronizing an oscillation signal produced by a burst mode DC-to-AC inverter driving illumination lamp(s) with the vertical synchronization signal provided to a liquid crystal display (LCD). In particular, the DC-to-AC inverter produces an oscillation signal (Vsawtooth) that controls the production of burst signals (Burst) used to drive the lamp(s). When Vsawtooth is above a reference voltage Vr (i.e., high), then a burst signal is produced. Conversely, when Vsawtooth is below Vr (i.e., low), no burst signal is produced. Each time a synchronization signal Sync is received, the oscillation signal Vsawtooth is pulled low for the duration of the synchronization signal, thereby causing the Burst signal (and lamps) to turn off or remain off. After the lapse of the synchronization signal, Vsawtooth resumes oscillating. In this manner, the cited reference at least somewhat synchronizes the oscillation signal Vsawtooth with the synchronization signal Sync.
The solution of the cited reference is less than satisfactory for more than one reason. First, the Vsawtooth waveform (and thus the lamp) is simply terminated during each vertical synchronization signal and restarted thereafter. Therefore, the lamp is turned on for a partial pulse duration (e.g., as shown in FIG. 2(d) of the cited reference) during each frame of data. This will affect the intensity of the data being displayed and can cause flicker. Further, UHP lamps, are very sensitive to the average DC current across their electrodes. High average DC currents are known to significantly shorten the lifetime of UHP lamps. With the present prior art driving circuit, the partial lamp pulse each frame can cause a DC imbalance, which can accumulate over time.
What is needed, therefore, is an illumination system that can eliminate flicker and/or fluctuations in brightness of a display. What is also needed is a system and method that minimizes any DC imbalance in the driving of light source lamps. What is also needed is a system and method that provides the foregoing advantages for systems with different display refresh rates.